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你在电梯里会不会变轻? – 译学馆
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你在电梯里会不会变轻?

Would you weigh less in an elevator? - Carol Hedden

假设你现在走进一间电梯。
You step into an elevator.
电梯开始极速下降。
It starts going down, fast.
这个时候你正好向上跳起的话将会发生什么呢?
What would happen if you jumped right when it started going down?
是你的头会撞到天花板吗?呵呵,想想就会觉得疼。
Would the ceiling hit your head? Ouch!
当电梯垂直下降时你会悬浮在空中吗?
Do you stay suspended in the air while the elevator plummets down?
让我们来一步一步的分析一下这个关于电梯的问题吧。
Let’s examine the elevator problem one step at a time.
首先,假设现在有一台记重仪。
First, consider a scale.
你知道的,就是那种你们平时用来测体重的那种。
You know, the kind of scale you weigh yourself on.
当你踏上去测量自己的体重时,
When you step on a scale to weigh yourself,
这时候产生了两种作用力。
there are two forces involved.
一个是重力将你往下拉。
One, gravity pulls you down.
另一个是测量仪承载你将往上推的力量。
Two, the scale pushes you up.
什么?你居然不知道测量仪能往上推?
What? You didn’t know a scale could push?
当然可以!如果它没有把你往上推,你会掉下去把地面打穿的。
Of course it can! If it didn’t push up on you, you would go crashing through the floor.
这股往上推的力我们称它为自然力,没错,它就这样理所当然的存在着。
This upward push is called the normal force and yes, it is normally there.
既然你只是一动不动地站在测量仪上。
Since you’re just standing there on the scale, you’re not moving.
因此,你没有加速度。
Therefore, you’re not accelerating.
牛顿第二运动定律表明,合力等于质量乘以加速度。
Newton’s second law of motion — net force equals mass times acceleration —
这就表明如果加速度等于零,那合力一定等于零。
tells us that if the acceleration equals zero, the net force must equal zero.
这同样意味着拉你向下的重力一定也等于推你向上的反作用力。
Which means that the force of gravity pulling you down must be equal to the force of the scale pushing you up.
了解到这些后,现在让我们来假设你正站在电梯里面的测重仪上面。
Now let’s suppose you’re standing on that scale in an elevator.
起初,电梯是静止的,
At first, the elevator is standing still,
所以你和测重仪也是静止的。
so you and the scale are standing still.
作用在你身上的两股力此时是大小相等方向相反的。
The two forces on you are equal and opposite.
你可以看一下测重仪是怎样艰难的将你托起的。
You can read how hard the scale is pushing by looking at it.
它上面显示的刻度我们称之为你的体重。
We call that your weight.
然后,电梯开始下降。
Then, the elevator starts falling down.
你和测重仪在电梯里面,所以你也在下降,
You and the scale are in the elevator, so you are falling down too,
越来越快。
faster and faster.
这意味着你开始有向下的加速度。
That means you are accelerating downward.
现在你的合力方向跟加速度的方向是相同的–向下
Now there is a net force in the same direction as the acceleration — down.
重力是不变的,那就说明体重秤给我们的力没有之前多了
Since gravity hasn’t changed, that must mean the scale isn’t pushing up as hard.
所以现在体重秤的数字比之前的要小
So the scale is reading a smaller number.
电梯下降的速度越快,体重秤就给我们的力越小
The faster the elevator accelerates, the less the scale pushes up.
那么如果你现在在电梯里跳呢?会停止下降吗?
What if you jumped? Would you stop falling?
电梯会碰到你的头吗?
Would the elevator hit your head?
那么,是什么力让电梯在下降?那就是—重力
Well, what’s pulling the elevator down? Gravity.
重力有拉着你么?那必须的啊
Is gravity pulling on you? Of course.
所以你跟电梯是相对静止的
So your relative position in the elevator will stay the same.
那么只有你能跳到天花板上它才回碰到你的头
The ceiling will only hit you if you can jump up to the ceiling.
现在思考:那电梯的加速度向上将会发生什么呢?
Now consider: what would happen if the elevator accelerated upward
还有更恐怖的,如果有个人偷偷的剪断了电梯的缆绳会怎样?
and, even more frightening, if someone snuck in and cut the cable holding the elevator,
那将会发生什么?
what would happen then?
考虑一下吧(๑• . •๑)
Think about it.

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